Avalanche and p-i-n photodiodes detect light by absorption of incident light and the detection of the free electrical charge generated in the absorption process. The optical reflectivity of the light entry surface is typically high, thus reducing the fraction of the incident light which actually enters the device and is absorbed. The most common approach to reduce that reflectivity has been to add an anti-reflection coating to the light entry surface.
In a number of materials, silicon in particular, the absorption length for a range of wavelengths is small. Thus, a significant fraction of the light in this wavelength range will not be absorbed before passing through the device. A partial solution to this problem has been to place a reflector on the surface opposed to the entry surface thus reflecting the light back through the sensitive region and doubling the path length for light absorption.
An alternative approach to reduce the incident surface reflectivity and to increase the light path length in the device has been to contour either the entry or back surface or both surfaces of the device. Haynos et al, Proceedings of the International Conference on Photovoltaic Power Generation, Hamburg, Germany, pp. 487-500, September, 1974, have disclosed a silicon photovoltaic solar cell in which a high density of tetrahedra with dimensions of about 2 microns in height and 2 microns at their base have been formed on the entry surface of the cell by chemically etching the surface using an anisotropic etchant. Light incident on this tetrahedrally shaped surface is partially transmitted and partially reflected. The portion reflected is then incident on a neighboring tetrahedron and is again partially transmitted and partially reflected. Thus, the incident light undergoes at least two interactions with the entry surface before leaving the device, thus reducing the device reflectivity.
Muller, Technical Digest of the International Devices Meeting, Washington, D.C., December, 1976, pp. 420-423, has disclosed a p-i-n photodiode having a sphere segment grating on the photodiode surface opposed to the light entry surface so that some of the light reflected from the back surface will be scattered at angles greater than the critical angle for total internal reflection and be trapped within the crystal. The sphere segments are pits about 3 microns in diameter and less than 1 micron deep resulting from etching though pinholes statistically distributed in a photoresist layer on the surface. Muller found that a considerable portion of light incident on such a grating is reflected at such an angle that it leaves the photodiode after only one reflection and thus did not provide significant improvement. Muller also disclosed an optimum grating, an asymmetrically grooved surface which is obtained by anisotropic etching of a surface oriented about 10.degree. off of a (111) plane.
These structures provide enhancement of the spectral response. However, in photoconducting devices where an external electrical field is applied, the surface contouring, particularly for the grooves and pyramids, can result in local regions of high electrical field which lead to nonuniformities in the spectral response of the device and to electrical breakdown and noise. Irregularities and nonuniformities in the surface contour will also lead to a nonuniform response across the surface of the device. It would be desirable to provide a photodiode having contoured surfaces without the presence of the nonuniformities and local regions of high electrical field present in the prior art devices.